1. Field of the Invention
This invention is generally in the field of medicine and cell culture, and in particular in the area of implantable organs formed on biocompatible artificial matrices.
2. Background of the Invention
Loss of organ function can result from congenital defects, injury or disease. Many times treatment with drugs or surgery is not in itself sufficient and the patient dies or is severely disabled. One approach for treatment has been to transplant donor organs or tissue into the patient. Drugs such as cyclosporin can be used to prevent tissue rejection. However, there is a tremendous shortage of donor organs, most of which must come from a recently deceased individual.
There have been a number of attempts to culture dissociated tissue and implant the cells directly into the body. For example, transplantation of pancreatic tissue, either as a whole organ or as a segment of an organ, into the diabetic patient has been attempted. Serum glucose appears to be controlled in a more physiological manner using this technique and the progression of complications is thereby slowed. An earlier approach which was not successful in achieving long-term benefits was the transplantation of islet cells through injection of isolated clusters of islet cells into the portal circulation, with implantation in the vascular bed of the liver. More recent methods have included encapsulation of pancreatic beta cells to prevent immune attack by the host and injection of fetal beta cells beneath the capsule of the kidney. Although there is evidence of short term function, long term results have been less satisfactory (D. E. R. Sutherland, Diabetologia 20, 161-185 (1981); D.E.R. Sutherland, Diabetologia 20, 435-500 (1981)). Currently whole organ pancreatic transplantation is the preferred treatment.
One of the problems with implanting dissociated cells into the body is that they do not form three dimensional structures and the cells are lost by phagocytosis and attrition. One approach to overcome this problem is wherein cells are encapsulated within spheres, then implanted. While this method can sometimes maintain viable functioning cells, the cells do not form organs or structures and rarely result in long term survival and replication of the encapsulated cells. Most cells have a requirement for attachment to a surface in order to replicate and to function.
The first attempts to culture cells on a matrix for use as artificial skin, which requires formation of a thin three dimensional structure, were described by Yannas and Bell in a series of publications, for example, U.S. Pat. Nos. 4,485,097; 4,485,096; 4,546,500; 4,060,081; 4,280,954; 4,458,678; and 4,505,266. They used collagen type structures which were seeded with cells, then placed over the denuded area. A problem with the use of the collagen matrices was that the rate of degradation is not well controlled. Another problem was that cells implanted into the interior of thick pieces of the collagen matrix failed to survive.
One method for forming artificial skin by seeding a fibrous lattice with epidermal cells is described in U.S. Pat. No. 4,485,097 to Bell, which discloses a hydrated collagen lattice that, in combination with contractile agents such as platelets and fibroblasts and cells such as keratinocytes, is used to produce a skin-equivalent. U.S. Pat. No. 4,060,081, to Yannas et al. discloses a multilayer membrane useful as synthetic skin which is formed from an insoluble non-immunogenic material which is nondegradable in the presence of body fluids and enzymes, such as cross-linked composites of collagen and a mucopolysaccharide, overlaid with a non-toxic material such as a synthetic polymer for controlling the moisture flux of the overall membrane. U.S. Pat. No. 4,458,678 to Yannas et al. discloses a process for making a skin-equivalent material wherein a fibrous lattice formed from collagen cross-linked with glycosaminoglycan is seeded with epidermal cells.
A disadvantage to the first two methods is that the matrix is formed of a "permanent" synthetic polymer. U.S. Pat. No. 4,458,678 has a feature that neither of the two prior patents has, a biodegradable matrix which can be formed of any shape, using the appropriate cells to produce an organ such as the skin. Unfortunately, there is a lack of control over the composition and configuration of the latter matrices since they are primarily based on collagen. Further, since collagen is degraded by enzymatic action as well as over time by hydrolysis, the degradation is quite variable.
U.S. Pat. No. 4,520,821 to Schmidt describes a similar approach that was used to make linings to repair defects in the urinary tract. Epithelial cells were implanted onto synthetic matrices, where they formed a new tubular lining as the matrix degraded. The matrix served a two fold purpose--to retain liquid while the cells replicated, and to hold and guide the cells as they
In U.S. Ser. No. 06/933,018, filed Nov. 20, 1986, now abandoned, entitled "Chimeric Neomorphogenesis of Organs Using Artificial Matrices" filed Nov. 20, 1986 by Joseph P. Vacanti and Robert S. Langer, a method of culturing dissociated cells on biocompatible, biodegradable matrices for subsequent implantation into the body was described. This method was designed to overcome a major problem with previous attempts to culture cells to form three dimensional structures having a diameter of greater than that of skin. Vacanti and Langer recognized that there was a need to have two elements in any matrix used to form organs: adequate structure and surface area to implant a large volume of cells into the body to replace lost function and a matrix formed in a way that allowed adequate diffusion of gases and nutrients throughout the matrix as the cells attached and grew to maintain viability in the absence of vascularization. Once implanted and vascularized, the porosity required for diffusion of the nutrients and gases was no longer critical.
However, even with the method described by Vacanti, the implant was initially constructed in vitro, then implanted. It is clearly desirable to be able to avoid the in vitro step. U.S. Ser. No. 07/343,158, filed Apr. 25, 1989, now abandoned, by Vacanti, et al., describes an approach used to address this problem. Recognizing the need for vascularization to maintain the implant in vitro, first addressed in the 1986 patent application U.S. Ser. No. 06/933,018, et al., the implant was seeded in vitro then immediately implanted into a highly vascularized tissue, the mesentery. A drawback with this was that the implant could only be made into this area of the body, and that a number of thin implants had to be used to achieve the requisite number of cells.
It is therefore an object of the present invention to provide an implant containing the requisite number of cells to replace lost organ function.
It is a further object of the present invention to provide a biocompatible, polymeric implant which can be implanted with cells without prior in vitro culturing and then degrades at a controlled rate over a period of time as the implanted cells replicate and form an organ structure.